Field of the Invention
The disclosures herein relate generally to antenna arrays, and more specifically, to electrically small antenna arrays.
Background of the Invention
When operated at “low” frequencies, traditional quarter-wavelength antennas become prohibitively large for certain applications. For example, a quarter-wavelength monopole operating at 10 MHz has a physical size of 7.5 m. This may be acceptable for an outdoor antenna (for instance), but would be impractical for a compact hand-held device. Thus, an antenna designer must employ electrically-small antenna (ESA) techniques in order to transmit and receive signals effectively using an antenna considerably smaller than this natural quarter-wavelength scale.
In short, there exists a significant need for advances in electrically small antennas, particularly arrays of electrically small antennas.
Magnetic antennas, particularly loopstick antennas, are often used for reception of low frequency signals. FIG. 1 is a circuit diagram showing a prior art loopstick antenna 104. Prior art loopstick antenna 104 comprises primary coil 131, ferrite rod 135, secondary coil 133 and tuning means 132. In alternate embodiments, ferrite rod 135 may be iron, some other ferromagnetic material, or even a non-ferromagnetic material to support primary coil 131. Ferrite rod 135 may also be a bar, cylinder, prism, or other geometric form suitable for supporting primary coil 131. In alternate embodiments ferrite rod may be dispensed with altogether if primary coil 131 is sufficiently stiff to maintain a suitable mechanical shape. Secondary coil 133 provides coupling into primary coil 131 and means by which prior art loopstick antenna 104 may further couple to an RF device.
FIG. 2 is a schematic diagram illustrating a prior art loopstick antenna 104. Primary coil 131 generally comprises turns with normals co-located with loopstick axis 205. Loopstick axis 205 is not only an axis of ferrite rod 135, but also a null axis for prior art loopstick antenna 104. Pattern of prior art loopstick antenna 104 lies generally along pattern axis 207. Pattern axis 207 is substantially orthogonal to loopstick axis 205.
Existing antenna arrangements are large and bulky or small and inefficient. Other existing antenna arrangements are prone to undesirable coupling to a mobile asset or person carrying an associated wireless device.